We have developed and validated in healthy subjects, a method to measure Dopamine D2 receptors not only in striatum, which has a high density of these sites, but also in extrastriatal regions (i.e. thalamus and medial temporal cortex), which has much lower density of these sites. These extrastriatal receptors are thought to be involved in the patho- physiology of psychosis. The technique involves constant infusion of radiotracer ([123Iepidepride) for approximately 30 hours. Infusions of radioactivity must be performed in a controlled hospital setting such as the GCRC.